The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal device of a reflection type and to a liquid crystal device of a reflection-transmission capacitance type.
In recent years, a liquid crystal display device of a reflection type capable of realizing a light, thin and low-power consumption type, among liquid crystal display devices using an active matrix, has attracted interest. The liquid crystal display device of a reflection type is characterized by its thinness, lightness and low-power consumption and is capable of realizing a display like paper because the display is attained by utilizing surrounding light. At the present time, a single polarizing plate system is put to practical use as a liquid crystal display of a reflection type. The liquid crystal display device of a reflection type using this system can have a high contrast and attain a relatively bright display because only one polarizing plate is used.
FIG. 1 is a diagram showing the panel structure of the above-mentioned liquid crystal display (LCD) device of a reflection type using the single polarizing plate system. In this panel structure, as shown in FIG. 1, liquid crystal of a TN type 9 is sandwiched between a transparent substrate 3, on one surface on which a phase difference plate 2 and a polarizing plate 1 are formed and on the other surface on which a transparent electrode 4 is formed, and a substrate 6 on the surface on which a diffuse reflective electrode 5 is formed. The liquid crystal of a TN type 9 is parallelly aligned at the boundary surface between the electrodes 4 and 5 and twisted in the direction of thickness. In this case, a dark state is displayed in a state in which a voltage is applied but, at this time, the molecules at the boundary surface are not deformed because of the anchoring effect, therefore, retardation occurs at this part and it is difficult to attain a very high contrast.
U.S. Pat. No. 4,701,028 has disclosed a liquid crystal display device using liquid crystal of a VA type 10, in which a ¼ wavelength plate 7 is used instead of the phase difference plate 2 and the boundary surface is vertically aligned as shown in FIG. 2. This can realize a dark state during the period with no voltage applied (in a state in which no alignment deformation exists). In this case, as no residual retardation exists in a black state, a very high contrast can be attained.
On the other hand, the optimization of the reflective electrode becomes very important in order to realize a bright display. For example, a technique for producing unevenness both randomly and very densely on the surface of the reflective electrode has been proposed. The object of this technique is to prevent the reflected light from being colored by preventing interference of light due to the repetitive pattern of the unevenness by increasing the extent of the randomness of the unevenness, and to reduce the components of regularly reflected light by decreasing the flat area by increasing the density of the unevenness. Further, a technique for attaining a bright display has been proposed, in which the average tilting angle of the unevenness is limited in order to condense the scattered light into an area within a certain range. Furthermore, Japanese Patent No. 3187369 has proposed a reflective electrode in which the probability of the existence of tilting angles within a specific range increases as the tilting angle increases, thereby a liquid crystal display element of a reflection type capable of obtaining a uniform brightness within the effective viewing angles has been realized.
The above-mentioned unevenness on the surface of the electrode was formed by the use of a photo lithography, but the process was complicated and there was a problem: the margin of manufacturing process was narrow because the reflection characteristics changed considerably when the shape was changed due to the exposure conditions.
The present applicants have developed a technique for forming a diffuse reflective electrode having wrinkle-like unevenness (microscopic grooves) without using a photo lithography in order to reduce the cost and have proposed a liquid crystal display device of a reflection type having a high reflectance and a high contrast ratio by adopting liquid crystal of a VA type in Japanese Unexamined Patent Publication (Kokai) No. 2002-221716.
Moreover, the present applicants have disclosed a technique in Japanese Unexamined Patent Publication (Kokai) No. 2002-296585 for controlling the orientation of the wrinkle-like unevenness by providing a structure 8 for generating a difference in level under the diffuse reflective electrode (wrinkle-like unevenness layer) 5 having the wrinkle-like unevenness as shown in FIG. 3. The surface of the wrinkle-like unevenness layer corresponds to the surface of the differences in level of the structures located thereunder. Various shapes of the structure are possible and an example is a linear structure parallel to the short side of a rectangular pixel electrode as shown in FIG. 4. As shown in FIG. 4, the domain defined by neighboring gate electrode lines 12 and neighboring source electrode lines 13 is the pixel domain and a pixel electrode 11 is provided therein. At the crossing of the gate electrode line 12 and the source electrode line 13, a TFT 14 is provided and the gate of the TFT 14 is connected to the gate electrode line 12, the source to the source electrode line 13, and the drain to the pixel electrode 11 via a contact hole 15. An auxiliary capacitor 16 is provided under the pixel electrode 11 and the pixel electrode 11 is connected to one of the electrodes of the auxiliary capacitor 16 via the contact hole 15.
On the other hand, the visibility is affected considerably by the light source environment in the case of the liquid crystal display device of a reflection type. Therefore in a dark environment, there arises a problem: the visibility is very poor. In the case of a liquid crystal display device of a transmission type, as a backlight is used, the power consumption is high but the contrast is high and the visibility is high in a dark environment. However, in a bright light source environment, the visibility is degraded considerably and the display quality becomes inferior to that of a reflection type.
As a system to solve the above-mentioned problem, a system of the combination of this liquid crystal of a reflection type and a front light (FL) or a reflective panel combining a semi-transmitting reflective film has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-333598.
However, in the case of the front light system, as colors are adjusted when the front light is lit, the whiteness degree (white balance) is degraded because of yellowing, and so on, in the case of a reflective display. This is because the color temperature of a fluorescent light is 4,200K to 5,500K and the color temperature of a light source in a normal light source environment is equal to or lower than 6,000K, which is the color temperature of the sunlight or, in other words, because the color temperature in a light source environment is low.
As to the semi-transmission system, a system in which the color purity is made to differ between the reflection domain and the transmission domain has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-267081 but, in this technique, the difference lies only in that a front light is replaced by a back light (BL) and colors are adjusted when the back light is lit, therefore the whiteness degree (white balance) is degraded because of yellowing, and so on, in the case of a reflection display.
In the above-mentioned systems, the reasons why the adjustment of the colors of the reflected light is difficult are that the color temperature of a light source is low and that the adjustment of the transmission spectra of the color filter (CF) in accordance with a low temperature light source is difficult.
FIG. 5 is a diagram of the pixel configuration of a TFT liquid crystal panel using three general RGB primary color filters (CF). As shown in FIG. 5, three sub-pixels, that is, an R pixel, a G pixel and a B pixel make up one display pixel. In the prior art, the CF pattern is formed so as to cover the entire display domain of sub-pixels (pixel electrode domain) and there are two cases, that is, one case where a gap exists between each CF as shown in FIG. 5 and the other case where a black matrix (BM) is formed in the gap.
FIG. 6A to FIG. 6D are diagrams for explaining the method for adjusting the chromaticity, and FIG. 6A and FIG. 6B show the case of a transmission type and FIG. 6C and FIG. 6D show the case of a reflection type, and in both cases the black matrix (BM) is provided. As shown in FIG. 6A and FIG. 6B, in the panel of a reflection type, three color filers (CF) 24R, 24G and 24B are provided on the CF substrate 3, a BM 25 is provided therebetween, and a transparent opposed electrode 21 is provided thereon. The opposed electrode 21 corresponds to the transparent electrode 4 in FIG. 1. A pixel electrode 22 is provided on the TFT substrate 6. The liquid crystal layers 9 and 10 are provided between the substrates and a back light (BL) source 26 is provided behind the TFT substrate 6. In the case of a transmission type, the chromaticity is adjusted by controlling the transmission characteristics of each color filter (CF) 24R, 24G and 24B. In the case of a general pigment-scattered CF (photo sensitive resin on which pigment is scattered is formed into patterns by a photo lithography), the thickness of the CF is the same as shown in FIG. 6A and the chromaticity characteristics are adjusted by controlling the amount of pigment to be scattered, or the amount of pigment to be scattered is the same as shown in FIG. 6B and the chromaticity characteristics are controlled by changing the thickness of film. Moreover, in addition to the chromaticity of the CF, it is possible to control the color balance by controlling the chromaticity of a BL light source.
As shown in FIG. 6C and FIG. 6D, in the panel of a reflection type, the three color filters (CF) 24R, 24G and 24B are provided on the CF substrate 3, the BM 25 is provided therebetween, and the transparent opposed electrode 21 is provided thereon. The TFT substrate 6 is provided with a reflective pixel electrode 23 corresponding to the reflective electrode 5 shown in FIG. 1. The liquid crystal layers 9 and 10 are provided between the substrates. In the panel of a reflection type also, the chromaticity is adjusted by controlling the transmission characteristics of each of the color filters (CF) 24R, 24G and 24B and the thickness of the CF is the same and the chromaticity characteristics are adjusted by controlling the amount of pigment to be scattered as shown in FIG. 6C, or the amount of pigment to be scattered is the same and the chromaticity characteristics are controlled by changing the film thickness as shown in FIG. 6D. Moreover, in the case of a reflection type, the color balance changes depending on the color temperature (chromaticity) of the external light.
Particularly in the case of a transmission type, it is usual that a light source whose color temperature is near to that of the D65 standard light source is used, but in a normal light source environment, light of a lower temperature than the D65 standard light source is predominant. Because of this, the color purity is degraded in the case of a reflection type. FIG. 7A and FIG. 7B show the color reproduction area plotted with R, G and B in the case of the D65 light source when the film thickness of the pigment-scattered type CF material is changed (the case where the amount of pigment to be scattered is changed is the same), and FIG. 7A shows the case of a transmission type and FIG. 7B shows the case of a reflection type. From FIG. 7A and FIG. 7B, it is found that the tendency toward saturation of the color purity in the G domain in the case of a reflection type is stronger than in the case of a transmission type. In the case of a reflection type, if the designed light source is the D65 environmental light source, there will not arise any problem but, in an actual environment, a light source equal to or lower than D55 is used in most cases and, therefore, the color reproduction area shifts in the direction toward lower temperatures (x and y increase and yellowing occurs). The above-mentioned tendency toward saturation is stronger in the case of a reflection type than in the case of a transmission type because the y value is prevented from being increased in order to avoid the above-mentioned phenomenon. In addition, this is because the transmission characteristics are prioritized in the case of a reflection type. Therefore, it was difficult to prevent the expansion of the color reproduction area (the NTSC ratio of the area of the RGB triangle) and the yellowing of the whiteness degree of a lower temperature light source in the case of a conventional reflection type.
FIG. 8 shows the change in the whiteness degree when the film thickness is changed in the panel of a reflection type. It was found that the whiteness degree is considerably influenced by yellowing (both x and y increase) when the light source changes from D65 to D55 in the standard reflective CF (film thickness is 0.75 μm) configuration. To prevent this, the color purity of the B sub-pixel is generally increased. However, only the y value is decreased and x shows a tendency to increase. Contrary to this, when the film thickness of RG is reduced, both x and y are found to show a tendency to decrease (shift toward high temperatures). However, it is found that when the film thickness of RG is reduced, the color reproduction area is reduced as shown in FIG. 9.
Moreover, the panel of a refection type excellent in display quality has been realized, but the display of a reflection type has been increasing its outdoor uses because of the recent mobile boom and the like. There arises a problem of the property of withstanding vibrations. Generally, the surface of a display or the like is unlikely to be pressed with strong force or used under constantly vibrational conditions, but it is necessary to prepare for a severely vibrational environment in which the display is used for mobile purposes or outdoor circumstances.
In the liquid crystal display device of a VA reflection type having the wrinkle-like unevenness and which has realized a high reflectance and a high contrast ratio, an afterimage appears and the display quality is degraded when the display of a moving cursor is attempted in a vibrating environment.
FIG. 10A to FIG. 10C are diagrams showing the alignment states of liquid crystal (sectional views) in the TFT driven liquid crystal display device of a VA type using n-type liquid crystal whose dielectric constant anisotropy is negative, wherein the occurrence and the change in the position of the disinclination line are shown. As shown in FIG. 10A, when the alignment control (control of the tilting orientation of liquid crystal) is not carried out, the tilting orientations of the liquid crystal molecules 10 between an opposed electrode 31 and a pixel electrode 32 are controlled by the oblique electric field generated at the edge of the pixel electrode 32 and a disinclination line (poor display part) 33 occurs at the domain where tilting orientations of the liquid crystal molecules meet. The disinclination line becomes unstable because of the external disturbance such as the unevenness on the electrode surface or the transverse electric field of drive wires (data bus, gate line) on the periphery of the pixel and the position of the disinclination line 33 changes, as shown in FIG. 10B and FIG. 10C, resulting in the degradation in display quality.
An example of a method for solving this problem is one in which a dielectric 34 is formed on the electrode (the opposed electrode 31, in this case) as shown in FIG. 11A, or a slit 35 is provided in the electrode (the opposed electrode 31, in this case) as shown in FIG. 11B so that the stability can be attained by controlling the electric field in the gap. As shown schematically, the disinclination line 33 is formed stably at the part of the protrusion 34 or the slit 35. However, when this method is used, as the disinclination line is formed in the effective display area, there arises a problem: the reflection or transmission characteristics are degraded.
In order to solve this problem, a method is possible in which a disinclination control structure (protrusion 34 or slit 35) is formed at the edge of the pixel electrode 32 as shown in FIG. 12A and FIG. 12B. By the use of this method, in a domain 36, the force for alignment control by the protrusion 34 and the edge of the pixel electrode 32 is strong and the disinclination line 33 is formed stably. In a domain 37, the force for alignment control by the edge of the pixel electrode 32 is strong. Owing to the forces for alignment control in the domains 36 and 37, normally the disinclination line 33 is formed stably as shown in FIG. 12A and FIG. 12B and no disinclination line is formed in other parts. Therefore, it is found that the degradation in the reflectance and the transmittance due to the disinclination can be prevented.
However, when vibrations are applied to the panel surface in the above-mentioned structure, it is found that the alignment was disturbed, many disinclination lines 38 occurred and the display quality is degraded in the domain between the domains 36 and 37, where the force for alignment control is weak, as shown in FIG. 12C.